The domestic automatic dishwasher is generally of the type having a washing chamber with open-framework racks therein for holding dishes to be washed and means for recirculating washing liquid accumulated in the lower end of the washing chamber upwardly over the dishes to loosen and carry away food soil therefrom. An inherent problem in such a machine is that food soil particles are suspended in the recirculating flow of washing liquid and that redeposition of these particles on the clean dishes can occur during the cleaning process. During the recirculation of the washing liquid large food particles flushed from the dishes will be carried downwardly and broken up into particularly small particles that are then washed back on to the dishes. These small particles adhere to the cleaned items and often defy removal during subsequent rinsing steps in the operation of the machine.
An approach to correcting this problem has been to provide a means to remove food particles from the recirculating flow so that the washing liquid moving downwardly in the washing chamber carries food soil with it but the same washing liquid redistributed upwardly in the washing chamber is relatively free of these soil particles. To accomplish this a filtering medium in the form of a screen has been interposed in the path of the liquid recirculation whereby soil particles are prevented from further passage while washing liquid is free to move therethrough and be recirculated in the machine's washing chamber. The use of a filtering screen introduces its own problem; that of the need for cleaning the collected soil from the screen either between each use of the dishwasher or between wash and rinse steps of the operational cycle so that the screen does not become clogged and thereafter prevent passage of liquid therethrough.
One approach to the filter-cleaning problem has been to provide a removable filter that the machine operator can take out of the machine, rinse in the sink, and then reinstall in the machine for further use. Another and more popular approach has been the provision of a self-cleaning filter wherein the filter is flushed by a reversed flow or an automatic filter rinsing step provided in the operational cycle of the machine. A notable example of a dishwashing machine having a self-cleaning filtering arrangement is disclosed in U.S. Pat. No. 2,629,391 issued to F. S. Hummel on Feb. 24, 1953. Hummel teaches the provision of a filtering screen disposed over the sump in the bottom of a dishwashing machine's wash chamber. He also teaches the use of a specific liquid injection step to flush soil from the filter and also to wash collected soil from the sump and outwardly through a gravity drain line.
More recent examples of self-cleaning filter arrangements in dishwashers will be found in U.S. Pat. No. 3,090,391 issued to H. J. Kaldenberg et al. on May 21, 1963 and U.S. Pat. No. 3,575,185 issued to D. J. Barbulesco on Apr. 20, 1971. The Kaldenberg et al. and Barbulesco patents relate to dishwashing machines utilizing an annular sump arrangement provided circumjacent the axial flow pumping mechanism in a dishwasher and having an annular screen filter arrangement disposed in close proximity to the sump. Each of these patents teaches a different structure for a means for slinging liquid outwardly toward the annular filter whereby a backwash is accomplished to remove soil particles therefrom.
Commonly assigned U.S. Pat. No. 3,807,419, issued to the inventors hereof and dated Apr. 30, 1974, teaches the combination of a self-cleaning filter arrangement with a soil receptacle disposed at a position remote from the wash chamber's sump. Drainage means specifically provide for draining particles from the receptacle during the drain cycle. This type of filtering system is referred to as a "bypass" or "partial-flow" system since only a portion of the washing liquid is filtered at any given time. At present washing volumes and pump rates, the total volume of liquid is recirculated through the spraying system approximately 20 times per minute. Therefore, it is reasonable to conclude that all of the liquid eventually passes through the "bypass" filter. Of course, one of the primary advantages of this system, in addition to not requiring manual filter cleaning, is that the dishwasher will continue to operate even if the filter becomes completely clogged. The filter is cleaned or backflushed by the downwardly cascading washing liquid which impinges against the downstream-side of the filter screen. The soil-collecting receptacle is placed adjacent the back wall of the washing chamber and receives for the most part, recirculating liquid which falls downwardly along the back wall. Reliance on the downwardly cascading liquid along the back wall to supply liquid to the receptacle may not be entirely satisfactory for every dishwasher design or recirculation system.